1. Field of the Invention
The present invention relates to an electronic camera, and more particularly relates to an electronic camera that speedily calculates exposure without using special exposure sensors.
2. Description of Related Art
In common silver-salt cameras, special sensors calculate the exposure based on the output of those sensors.
In order to design lower cost electronic cameras, the exposure is calculated from the output of the imaging element without providing special sensors.
FIG. 7 is one example of exposure calculation operation. In step S61, operation is in standby until an unspecified period of time has elapsed. For example, after the electrical power source is turned on from an imaging element such as a CCD in FIG. 8, when an NTSC method is used, a readout operation is performed on the image data of each frame at 1/30th of a second per frame. However, the CCD output is unstable for the initial three frames. Therefore, there is a standby time (i.e., waiting time) from the first through third frames in order to wait for the CCD output to stabilize.
When the unspecified period of time has elapsed, operation advances to step S62 to set the preset default exposure amount. For example, in the event the stop is fixed, a prescribed shutter speed is set.
In step S63, exposure is performed only for the time corresponding to the shutter speed setting. That is, the electrical load that occurs in the period of time corresponding to the shutter speed is produced and a readout is performed in step S64. In order to perform the exposure and readout in one frame-length components (as in FIG. 8), when the exposure (step S63) is performed in a prescribed frame (e.g., the third frame), the readout (step S64) is performed in the next frame (e.g., the fourth frame).
In step S65, the signal level readout from step S64 is judged to determine whether it is the optimum value. If it is not an optimum value, operation advances to step S66 and a preset added value α is added to the value set in step S62 and the new value is a new set value. Then, operation returns to step S63 and exposure is performed corresponding to the new set value.
The calculation for determining the new set value is performed in the frame (e.g., the fifth frame) that follows the readout frame.
This operation is performed in repetition until the readout signal level from step S65 is judged to be at an optimum value and then the photography preparation is complete.
The optimum exposure value is generally determined in conventional electronic cameras in three operations, namely an exposure process, a readout process and a calculation process. However, these processes are time-consuming. Further, in video cameras, it is rare to take in only one frame of an image so that even though the process to calculate the exposure, in order to photograph images of at least several seconds or more in length, took time, it did not present a practical problem. However, problems exist in electronic cameras where the image is basically a one-frame image so that when it takes time for the exposure calculation to be done on several tens of frames, the photography of images is troublesome.